Fossil PuzzleSolutions:

Welcome to our Puzzle Fossil -- this one from Jackson County, Tennessee. 

What is it -- an egg?
An eye?  A fossilized bug?

It's none of those things.  In fact, it is a familiar member of our family of Tennessee fossils, and it's an easy one to identify one you look at it the right way!

Our first clue comes from its location.  Jackson County is home to landforms deposited during the Ordovician period over 400 million years ago -- long before eggs or bugs or other familiar animals evolved.  During this time, Tennessee was a land of water -- covered by a shallow salt-water sea that was home to trilobites, rugose corals, sponges, and underwater  Gardens of crinoids -- flower-like filter-feeding animals were anchored to the sea floor by long, flexible stalks. 


The hard parts are built from a form of calcium carbonate called "Calcite."  Crinoid stems are common fossils in Tennessee, although they are sometimes mistaken for fossilized worms, or called "Indian money" because they break apart into coin-like cylinders.  Because this cylinder was broken at a diagonal angle, it has an oval appearance that at first glance makes it resemble a human eye.


Taking another look at our fossil, we can see that it appears to be built from a stack of flat disks.  The outer parts had a spiculated matrix that has dissolved, leaving an inner layer of calcite sometimes surrounding a center hole that can have a pentagon shape.  That five-sided structure resists kinking while still allowing an element of flexibility, and identifies it as an echinoderm.

An interesting fact about echinoderms is that they follow embryonic development pathways that classify them as deuterostomes -- linking them to vertebrate animals like humans.  This suggests that starfish, sand dollars and crinoids could be part of our own evolutionary line -- unlike worms, clams, insects or crustaceans which are protostomes.  

So our mystery fossil turns out to be one of our own cousins-- a remote and unfamiliar ancestor, entombed in rock long ago.  Thanks Wanda for sending in this interesting fossil!

Sand Dollar The sand dollar is another familiar five-sided echinoderm that we might find on a modern beach.